System and method for spread spectrum communication

ABSTRACT

A spreading code generating apparatus carries out M-sequence multiplication and zero value addition of generated orthogonal codes to obtain channel identification codes, and then carries out inverse Fourier transform of the channel identification codes to generate spreading codes and inverse spreading codes. A transmitting apparatus and a receiving apparatus store the spreading codes and the inverse spreading codes generated by the spreading code generating apparatus, respectively, and use a single code respectively selected from the stored codes to carry out spread spectrum communication.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to a system and a method for spread spectrumcommunication for spreading an input signal over a wide frequency bandfor communication.

2. Background art

In conventional spread spectrum communication using Direct Sequence (DS)spreading schemes, orthogonal codes such as Walsh codes, Hadamard codes,and Gold codes, or PN (Pseudorandom Noise) codes are used as spreadingcodes. For example, Japanese Patent No. 2929244 discloses two types ofPN codes for use as the spreading codes, and Japanese Patent PublicationNo. 6-91509 discloses codes generated by combining two or more types ofPN codes for use as the spreading codes. As such, in the conventionalspread spectrum communication, spreading codes orthogonal to each otheron the time axis are used.

However, the spreading codes used for conventional spread spectrumcommunication are orthogonal to each other only on the time axis, andthe electric power characteristics of a spread signal are not uniform onthe frequency axis. Therefore, in the conventional spread spectrumcommunication, transmission characteristics and multiplexing capacityhave certain limitations.

Therefore, an object of the present invention is to provide a spreadspectrum communication system and a spread spectrum communication methodthat are superior to conventional ones in transmission characteristicsand multiplexing capacity.

SUMMARY OF THE INVENTION

To achieve the object mentioned above, the present invention has thefollowing aspects.

A first aspect of the present invention is directed to a communicationsystem for carrying out spread spectrum communication, including:

-   -   a first storage section for storing a spreading code;    -   a second storage section for storing an inverse spreading code        corresponding to the spreading code stored in the first storage        section;    -   a spreading section for spreading an input signal using the        spreading code stored in the first storage section; and    -   an inverse spreading section for inversely spreading a signal        outputted from the spreading section, wherein    -   the spreading code stored in the first storage section is        included in a vector group obtained by carrying out inverse        Fourier transform of a plurality of different channel        identification codes.

In the first aspect, the different channel identification codes aresubjected to inverse Fourier transform, and the resultant codes are usedas spreading codes. When these spreading codes are subjected to Fouriertransform, absolute values of the resultant elements are constant.Therefore, with the use of such spreading code for spreading, the inputsignal is spread with uniform power on the frequency axis. Such spectrumspreading with uniform power on the frequency axis improves transmissioncharacteristics and increases multiplexing capacity.

According to a second aspect of the present invention based on the firstaspect, each element of the spreading code stored in the first storagesection is a complex number, and each element of the inverse spreadingcode stored in the second storage section is a complex number,

-   -   the spreading section carries out complex multiplication of the        input signal and each element of the spreading code stored in        the first storage section, and    -   the inverse spreading section carries out complex multiplication        of the signal outputted from the spreading section and each        element of the inverse spreading code stored in the second        storage section.

In the second aspect, the input signal is spread by using the spreadingcode whose elements are complex numbers, and the spread signal isinversely spread by using the inverse spreading code whose elements arecomplex numbers.

According to a third aspect of the present invention based on the secondaspect, the communication system further includes a modulating sectionfor carrying out quadrature modulation of a carrier by using the signaloutputted from the spreading section, and

-   -   a demodulating section for carrying out quadrature demodulation        of a signal outputted from the modulating section, and        outputting the demodulated signal to the inverse spreading        section.

In the third aspect, the carrier is subjected to quadrature modulationby using the real part and the imaginary part of the spread signal, andthe modulated carrier is then subjected to quadrature demodulation.Thus, it is possible to reproduce the spread signal by the receivingapparatus.

According to a fourth aspect of the present invention based on the thirdaspect, each element of the inverse spreading code stored in the secondstorage section is a conjugate complex number of a corresponding elementof the spreading code stored in the first storage section.

In the fourth aspect, the spread signal is inversely spread by using theinverse spreading code which is the conjugate number of the spreadingcode. Thus, the coordinate axis which was rotated by a predeterminedamount at the time of spreading can be returned to its original positionat the time of inverse spreading.

According to a fifth aspect of the present invention based on the firstaspect, the communication system further includes a filter section foroutputting low-frequency components of a signal outputted from theinverse spreading section.

In the fifth aspect, it is possible to produce an output signalcorresponding to the input signal.

According to a sixth aspect of the present invention based on the firstaspect, the communication system further includes a synchronizationtracking section for finding a synchronization point for the inversespreading code stored in the second storage section by detecting a peakpoint of a signal outputted from the inverse spreading section.

In the sixth aspect, the synchronization point of the spread signal canbe found by the receiving apparatus.

According to a seventh aspect of the present invention based on thefirst aspect, the input signal supplied to the spreading section is acomplex number composed of a real part and an imaginary part.

In the seventh aspect, it is possible to transmit an input signalsubjected to baseband modulation in the QPSK (Quadrature Phase ShiftKeying) scheme, the 16 QAM (Quadrature Amplitude Modulation) scheme, orthe like.

According to an eighth aspect based on the first aspect, the channelidentification codes are numerical value sequences of orthogonal codes.

According to a ninth aspect based on the first aspect, the channelidentification codes are numerical value sequences of Walsh codes.

According to a tenth aspect based on the first aspect, the channelidentification codes are numerical value sequences of Hadamard codes.

According to an eleventh aspect based on the first aspect, the channelidentification codes are numerical value sequences of Gold codes.

In the eighth to eleventh aspects, by carrying out inverse Fouriertransform of orthogonal codes, Walsh codes, Hadamard codes, or Goldcodes, it is possible to find spreading codes orthogonal to each otheron the frequency axis.

According to a twelfth aspect of the present invention based on thefirst aspect,

-   -   the channel identification codes are numerical value sequences        obtained by multiplying numerical value sequences of orthogonal        codes by an M-sequence.

In the twelfth aspect of the present invention, the numerical valuesequence obtained by multiplying the orthogonal code by the M-sequenceis subjected to inverse Fourier transform. Thus, it is possible to findspreading codes orthogonal to each other on the frequency axis. Also, itis possible to randomize a bit pattern of the channel identificationcodes.

According to a thirteenth aspect of the present invention based on thefirst aspect, the channel identification codes are numerical valuesequences obtained by adding or inserting a predetermined number of zerovalues at a same position in different numerical value sequences.

In the thirteenth aspect, the numerical value sequence obtained byadding or inserting a predetermined number of zero values at the sameposition in different numerical value sequences is subjected to inverseFourier transform. Thus, when the spread signal is transformed on thefrequency axis, it is possible to free up a frequency band as an unusedfrequency band. In one example, by adding zero values at the head andthe tail of each numerical value sequence, a guard band can be providedbetween two spread signals on the frequency axis, thereby simplifyingthe construction of the filter of the receiving apparatus for extractinga required band. In another example, by inserting zero values in themiddle of each numerical value sequence, a carrier hole can be providedon the frequency axis, thereby preventing another system frominterfering with a carrier of the frequency band.

A fourteenth aspect of the present invention is directed to atransmitting apparatus for spread spectrum communication. Thetransmitting apparatus includes:

-   -   a storage section for storing a spreading code; and    -   a spreading section for spreading an input signal by using the        spreading code stored in the storage section, wherein    -   the spreading code is included in a vector group obtained by        carrying out inverse Fourier transform of a plurality of        different channel identification codes.

In the fourteenth aspect, a transmitting apparatus for carrying outspectrum spreading with uniform power on the frequency axis can bestructured. By carrying out spectrum communication by using thistransmitting apparatus and a receiving apparatus corresponding thereto,the transmission characteristics are improved, and multiplexing capacityis increased.

A fifteenth aspect of the present invention is directed to a receivingapparatus for spread spectrum communication. The receiving apparatusincludes:

-   -   a storage section for storing an inverse spreading code        corresponding to a spreading code; and    -   an inverse spreading section for inversely spreading a signal        that has been spread with the spreading code, by using the        inverse spreading code stored in the storage section, wherein    -   the spreading code is included in a vector group obtained by        carrying out inverse Fourier transform of a plurality of        different channel identification codes.

In the fifteenth aspect, a receiving apparatus for carrying out spectrumspreading with uniform power on the frequency axis can be structured. Bycarrying out spectrum communication by using this receiving apparatusand a transmitting apparatus corresponding thereto, the transmissioncharacteristics are improved, and multiplexing capacity is increased.

A sixteenth aspect of the present invention is directed to acommunication method for carrying out spread spectrum communication. Themethod includes the steps of:

-   -   spreading an input signal by using a spreading code;    -   transmitting the signal spread in the spreading step;    -   receiving the signal transmitted in the transmitting step; and    -   inversely spreading the signal received in the receiving step by        using an inverse spreading code corresponding to the spreading        code, wherein    -   the spreading code is included in a vector group obtained by        carrying out inverse Fourier transform of a plurality of        different channel identification codes.

A seventeenth aspect of the present invention is directed to atransmitting method for spread spectrum communication. The transmittingmethod includes the steps of:

-   -   spreading an input signal by using a spreading code; and    -   transmitting the signal spread in the spreading step, wherein    -   the spreading code is included in a vector group obtained by        carrying out inverse Fourier transform of a plurality of        different channel identification codes.

An eighteenth aspect of the present invention is directed to a receivingmethod for spread spectrum communication. The receiving method includesthe steps of:

-   -   receiving a signal that has been spread with a spreading code;        and    -   inversely spreading the signal received in the receiving step by        using an inverse spreading code corresponding to the spreading        code, wherein    -   the spreading code is included in a vector group obtained by        carrying out inverse Fourier transform of a plurality of        different channel identification codes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the configuration of acommunication system according to an embodiment of the presentinvention;

FIG. 2 is an illustration showing a process of generating spreadingcodes in the communication system according to the embodiment of thepresent invention;

FIG. 3 is an illustration showing one example of channel identificationcodes and spreading codes in the communication system according to theembodiment of the present invention;

FIG. 4 is an illustration showing a detailed configuration of thecommunication system according to the embodiment of the presentinvention;

FIG. 5 is an illustration showing one example of transmissioncharacteristics of the communication system according to the embodimentof the present invention; and

FIGS. 6A-6C are a block diagrams showing other configurations of thespreading code generating apparatus according to the embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram illustrating the configuration of acommunication system according to one embodiment of the presentinvention. The communication system according to the present embodimentincludes a transmitting apparatus 20 and a receiving apparatus 30,between which spread spectrum communication is carried out. Beforecommunication is carried out, a spreading code generating apparatus 10generates spreading codes 121 to be used in the transmitting apparatus20 and an inverse spreading codes 131 to be used in the receivingapparatus 30. The spreading code generating apparatus 10 includes anorthogonal code generating section 11, an M-sequence generating section12, a multiplier 13, a zero value adding section 14, and an inverseFourier transforming section 15. The transmitting apparatus 20 includesa spreading code storage section 21, a spreading section 22, a filtersection 23, an RF (Radio Frequency) modulating section 24, and anantenna 25. The receiving apparatus 30 includes an antenna 31, an RFdemodulating section 32, an inverse spreading code storage section 33,an inverse spreading section 34, a filter section 35, and asynchronization tracking section 36.

First, the operation of the spreading code generating apparatus 10 isdescribed. Before spread spectrum communication is carried out betweenthe transmitting apparatus 20 and the receiving apparatus 30, thespreading code generating apparatus 10 operates as follows. Theorthogonal code generating section 11 generates an orthogonal code thatincludes a predetermined number of vectors having a predetermined lengthand containing elements each indicating 1 or −1. Examples of suchorthogonal codes are Hadamard codes, Walsh codes, and Gold codes.

The M-sequence generating section 12 generates an M-sequence (maximumlength sequence) having the same length as the vector generated by theorthogonal code generating section 11 and containing elements eachindicating 1 or −1. To generate an M-sequence having an arbitrarylength, an m-stage shift register is used to find an M-sequence having alength of (2^(m)−1), and the found M-sequence is repeated as required.

The multiplier 13 multiplies, element by element, each vector includedin the orthogonal codes generated by the orthogonal code generatingsection 11 by the M sequence generated by the M-sequence generatingsection 12. Specifically, consider a case where a vector of anorthogonal code generated by the orthogonal code generating section 11is (A₁, A₂, . . . , A_(n)) and the M-sequence generated by theM-sequence generating section 12 is (B₁, B₂, . . . , B_(n)). In thiscase, the multiplier 13 outputs (A₁B₁, A₂B₂, . . . , A_(n)B_(n)) as aproduct of both elements. This M-sequence multiplication is to randomizea bit pattern of the channel identification code, which is describedfurther below.

The zero value adding section 14 adds a predetermined number of zerovalues to the head and the tail of each vector outputted from themultiplier 13. The zero value adding section 14 may add one or more zerovalues to either one or both of the head and the tail of each vector.When zero values are added to both the head and the tail of each vector,the number of zero values to be added to the head may be equal to thenumber added to the tail, or may be different therefrom. This zero valueaddition is to provide a guard area between two spread signalstransformed on the frequency axis, thereby simplifying the structure ofa filter of the receiving apparatus 30 for extracting a required band.The zero-value-added vectors are different from each other, andtherefore are used as channel identification codes for spread spectrumcommunication.

The inverse Fourier transforming section 15 carries out inverse Fouriertransform of each channel identification code obtained by the zero valueadding section 14 to find a vector having the same number of elements asthe original vector. A group formed by the found vectors is called “avector group”. The elements of each vector after inverse Fouriertransform become complex numbers. All or part of the vectors included inthe vector group obtained by the inverse Fourier transforming section 15are used by the transmitting apparatus 20 as spreading codes 121.Further, the elements of the found vectors are replaced with theirrespective conjugate complex numbers, thereby obtaining inversespreading codes 131 to be used by the receiving apparatus 30.

With reference to FIG. 2, the operation of the spreading code generatingapparatus 10 is exemplarily described. The orthogonal code generatingsection 11 generates, for example, a Hadamard code each includingsixteen vectors having a length of 16 (a vector group VG, illustrated inFIG. 2). The M-sequence generating section 12 generates, for example, anM-sequence having a length of 16 such as (1, −1, 1, 1, −1, 1, 1, −1, 1,−1, −1, 1, −1, 1, −1, −1). The multiplier 13 multiplies, element byelement, the respective elements of the sixteen vectors generated by theorthogonal code generating section 11 by the M-sequence generated by theM-sequence generating section 12 to output a vector group VG_(b)illustrated in FIG. 2. The zero value adding section 14 adds two zerovalues to each of the sixteen vectors outputted from the multiplier 13both at the head and the tail to output a vector group including sixteenvectors having a length of 20 (a vector group VG_(c) illustrated in FIG.2). The vectors included in the vector group VG_(C) are used as thechannel identification codes.

The inverse Fourier transforming section 15 carries out inverse Fouriertransform of the sixteen channel identification codes obtained by thezero value adding section 14. FIG. 3 is an illustration showing oneexample of the channel identification codes and the spreading codes inthe spreading code generating apparatus 10. FIG. 3 illustrates how avector V_(c1) illustrated in FIG. 2 is transformed to a vector V_(d1)illustrated in FIG. 2 through inverse Fourier transform. The inversetransforming section 15 carries out inverse Fourier transform of therespective sixteen channel identification codes to obtain a vector groupincluding sixteen vectors each having a length of 20 and containingelements being complex numbers (a vector group VG_(d) illustrated inFIG. 2). All or part of the sixteen vectors included in the vector groupVG_(d) are used by the transmitting apparatus 20 as the spreading codes121. The elements of the found vectors are replaced with theirrespective conjugate complex numbers, thereby obtaining the inversespreading codes 131 to be used by the receiving apparatus 30.

Next, the operations of the transmitting apparatus 20 and the receivingapparatus 30 are described. Before communication is carried out, thetransmitting apparatus 20 and the receiving apparatus 30 are suppliedwith the spreading codes 121 and the inverse spreading codes 131,respectively, by the spreading code generating apparatus 10. In thetransmitting apparatus 20, the spreading codes 121 supplied by thespreading code generating apparatus 10 are stored in the spreading codestorage section 21. In the receiving apparatus 30, the inverse spreadingcodes 131 supplied by the spreading code generating apparatus 10 arestored in the inverse spreading code storage section 33. The spreadingcodes 121 and the inverse spreading codes 131 may be stored in therespective storage sections at the time of manufacturing, or may besupplied through input means or communication means after manufacturing.Note that the number of codes stored by the spreading code storagesection 21 and the inverse spreading code storage section 22 may be one,respectively.

For communication between the transmitting apparatus 20 and thereceiving apparatus 30, the spreading code storage section 21 selectsone spreading code 122 from the stored spreading codes 121 for output.Similarly, the inverse spreading code storage section 33 selects oneinverse spreading code 132 from the stored spreading codes 131 foroutput. In more detail, the spreading code storage section 21 outputs,as the spreading code 122 corresponding to a channel to be used, aspreading code through inverse Fourier transform of a channelidentification code for that channel. The inverse spreading code storagesection 33 outputs, as the inverse spreading code 132 corresponding to achannel to be used, an inverse spreading code obtained through inverseFourier transform of a channel identification code for that channel.

The spreading section 22 is supplied with an input signal 201 as asignal to be transmitted. The input signal 201 is a signal obtained bysubjecting error correction encoding and predetermined basebandmodulation to data to be transmitted. The spreading section 22 spreadsthe input signal 201 with the spreading code 122 stored in the spreadingcode storage section 21, and then outputs a spread signal 202. In moredetail, the spreading section 22 is supplied with the elements of thespreading code 122 at every time interval T2, which is shorter than atime interval T1 during which the input signal 201 is changed. Thespreading section 22 multiplies the input signal 201 by each element ofthe spreading code 122 at every time interval T2. Such multiplication ofeach element of the spreading code at every time interval T2 isgenerally called “multiplication of the spreading code at a chip rate”.

In the present embodiment, each element of the spreading code 122 is acomplex number. Therefore, the spreading section 22 multiplies the inputsignal by the spreading code 122 at a chip rate. That is, the spreadingsection 22 carries out complex multiplication of the input signal 201and each element of the spreading code 122 at every time interval T2.For example, consider a case where an element of the spreading code 122is (C+Dj) when the imaginary unit is j. When the input signal 201indicates a real number A, the spreading section 22 outputs a signalindicative of A×(C+Dj)=(AC+ADj) as the spread signal 202. When the inputsignal 201 indicates a complex number (A+Bj), the spreading section 22outputs a signal (A+Bj)×(C+Dj)={(AC−BD)+(AD+BC) j} as the spread signal202. With such operation of the spreading section 22, the input signal201 is spread over a wide frequency band on the frequency axis.

The filter section 23 is implemented by using a band-pass filter. Thefilter section 23 eliminates noise components included in an unwantedband from the spread signal 202 outputted from the spreading section 22.The RF modulating section 24 carries out quadrature modulation of acarrier with a signal outputted from the filter section 23, and outputsa signal of a radio frequency. In more detail, the RF modulating section24 modulates the carrier with the real part of the signal outputted fromthe filter section 23 to a first direction, and also modulates thecarrier with the imaginary part of the signal outputted from the filtersection 23 to a second direction that is different from the firstdirection in phase by 90 degrees. The antenna 25 emits a radio wavebased on the signal outputted from the RF modulating section 24.

In the receiving apparatus 30, the antenna 31 receives the radio waveemitted from the transmitting apparatus 20, and outputs a signal of aradio frequency. The RF demodulating section 32 carries out orthogonaldemodulation of the signal emitted from the antenna 31 to output abaseband signal as a demodulated signal 301. In more detail, the RFdemodulating section 32 demodulates the signal emitted from the antenna31 in the above-mentioned first direction to find the real part and alsoin the above-mentioned second direction to find the imaginary part, andthen outputs the demodulated signal 301 in complex number form.

The inverse spreading section 34 uses the inverse spreading code 132stored in the inverse spreading code storage section 33 to carry outinverse spreading of the demodulated signal 301, and then outputs theinverse spread signal 302. In more detail, the inverse spreading section34 carries out complex multiplication of the demodulated signal 301 andthe inverse spreading code 132 at the chip rate. That is, the inversespreading section 34 carries out complex multiplication of thedemodulated signal 301 and each element of the inverse spreading code132 at every time interval T2. For example, consider a case where anelement of the inverse spreading code 132 is a complex number (C−Dj) andthe demodulated signal indicates a complex number (E+Fj). In this case,the inverse spreading section 34 outputs a signal(E+Fj)×(C−Dj)={(EC+FD)+(−ED+FC) j} as the inverse spread signal 302.With such operation of the inverse spreading section 34, the inputsignal 201 spread over the wide area on the frequency axis is convergedto a specific frequency.

The filter section 35 is implemented by using a low-pass filter. Thefilter section 35 eliminates noise components included in an unwantedband from the inverse spread signal 302 outputted from the inversespreading section 34, and outputs an output signal 303. Thus, thereceiving apparatus 30 can produce the output signal 303 correspondingto the input signal 201. As the input signal 201 is obtained from datato be transmitted by encoding for error correction and basebandmodulation, the output signal 303 is subjected to reverse processing(that is, baseband demodulation and decoding for error correction).

The synchronization tracking section 36 outputs a synchronizing signal311 to the inverse spreading code storage section 33. The inversespreading code storage section 33 outputs the inverse spreading code 132to the inverse spreading section 34 in synchronization with thesynchronizing signal 311 outputted from the synchronization trackingsection 36. By appropriately changing output timing of the synchronizingsignal 311, the synchronization tracking section 36 detects timing whenthe signal outputted from the inverse spreading section 34 becomesmaximum in level (the timing is referred to as a peak point). With suchoperation of the synchronization tracking section 36, the receivingapparatus 30 can find optimum timing for inverse spreading processing.Note that the receiving apparatus 30 may further include an AFC(Automatic Frequency Control) circuit for establishing synchronizationin frequency with the transmitting apparatus 20.

With reference to FIG. 4, main components of the transmitting apparatus20 and the receiving apparatus 30 are described in detail. In FIG. 4,the spreading section 22, the RF modulating section 24, the RFdemodulating section 32, the inverse spreading section 34, the filtersection 35, and the synchronization tracking section 36 are illustratedin detail. Hereinafter, a case where the input signal 201 has beenbaseband-modulated with the QPSK (Quadrature Phase Shift Keying) schemeis exemplarily described. In this case, the spreading code 122, theinverse spreading code 132, the input signal 201, and the demodulatedsignal 301 are all complex numbers. Therefore, the spreading code 122 isdenoted as (C+Dj), the inverse spreading code 132 as (C−Dj), the inputsignal 201 as (A+Bj), and the demodulated signal 301 as (E+Fj). In thiscase, A and B are 1 or −1, and C, D, and E are arbitrary real numbers.

The spreading section 22 includes four multipliers (first to fourth), asubtractor, and an adder. The first multiplier finds a product AC of thereal part A of the input signal 201 and the real part C of the spreadingcode 122. The other three multipliers find a product BD, a product AD,and a product BC, respectively. The subtractor subtracts the product BDfrom the product AC to find a real part (AC−BD) of the spread signal202. The adder adds the product AD and the product BC together to findan imaginary part (AD+BC) of the spread signal 202. The spreadingsection 22 outputs the real part and the imaginary part of the spreadsignal 202 separately.

The RF modulating section 24 includes two multipliers and an adder. Onemultiplier multiplies the real part of the spread signal 202 by anin-phase carrier (cosine wave), while the other multiplies the imaginarypart of the spread signal 202 by an orthogonal carrier (sine wave). Theadder adds signals outputted from the two multipliers together. The RFmodulating section 24 outputs a single signal as a result of quadraturemodulation of the carriers. Generally, in QPSK modulation, orthogonalcarrier modulation is performed with four values (1, 1), (1, −1), (−1,1), and (−1, −1). In the present embodiment, however, values other thanthe above four values can be used for orthogonal carrier modulation.

The RF demodulating section 32 includes two multipliers and two low-passfilters (denoted as LPF in FIG. 4). The signal outputted from the RFmodulating section 24 is divided into two in the RF demodulating section32. One multiplier multiplies one of the divided signals by the in-phasecarrier (cosine wave), while the other multiplier multiplies the otherby the orthogonal carrier (sine wave). The two low-pass filtersrespectively eliminate noise components included in an unwanted bandfrom the signals outputted from the two multipliers. With such operationof the RF demodulating section 32, the demodulated signal 301corresponding to the spread signal 202 is produced. The RF demodulatingsection 32 outputs the real part and the imaginary part of thedemodulated signal 301 separately.

The inverse spreading section 34 includes four multipliers (first tofourth), a subtractor, and an adder. The first multiplier finds aproduct EC of the real part E of the demodulated signal 301 and the realpart C of the inverse spreading code 132. The other three multipliersfind a product (−FD), a product (−ED), and a product FC, respectively.The subtractor subtracts the product (−FD) from the product EC to find areal part (EC+FD) of the inverse spread signal 302. The adder adds theproduct (−ED) and the product FC together to find an imaginary part(−ED+FC) of the inverse spread signal 302. The inverse spreading section34 outputs the real part and the imaginary part of the inverse spreadsignal 302 separately.

The filter section 35 includes two accumulators. One accumulatoraccumulates the real part of the inverse spread signal 302 outputtedfrom the inverse spreading section 34 for one data symbol of the inputsignal 201, and outputs a real part A′ of the output signal 303.Similarly, the other accumulator accumulates the imaginary part of theinverse spread signal 302 outputted from the inverse spreading section34 for one data symbol of the input signal 201, and outputs an imaginarypart B′ of the output signal 303. The filter section 35 outputs the realpart and the imaginary part of the output signal 303 separately.

The synchronization tracking section 36 includes a correlation valuecalculating unit 37 and a peak detecting unit 38. The correlation valuecalculating unit 37 accumulates the inverse spread signal 302 for onedata symbol of the input signal 201, with the real part and theimaginary part of the inverse spread signal 302 being separate from eachother. That is, the correlation value calculating unit 37 finds acomplex number represented by {Σ(EC+FD)+Σ(−ED+FC)j} (where Σ is a sumfor one data symbol of the input signal 201). The correlation valuecalculating unit 37 then calculates an absolute value of the foundcomplex number for output to the peak detecting unit 38 as a correlationvalue. The peak detecting unit 38 outputs the synchronizing signal 311to the inverse spreading code storage section 33 as appropriatelychanging output timing. When the demodulated signal 301 and the inversespreading code 132 become synchronized with each other, the correlationvalue outputted from the correlation value calculating unit 37 becomesmaximum. The peak detecting unit 38 detects, as a peak point, timing inwhich the correlation value outputted from the correlation valuecalculating unit 37 becomes maximum, and outputs the synchronizingsignal 311 at the detected peak point.

In the above description, the input signal 201 is assumed to have beenbaseband-modulated with the QPSK scheme. It is also possible, however,to construct a communication system similar in configuration to thesystem illustrated in FIG. 4 in a case where the input signal 201 hasbeen baseband-modulated with another quadrature modulation scheme. Forexample, in a case where the input signal 201 has beenbaseband-modulated with 16 QAM (Quadrature Amplitude Modulation) scheme,the real part A and the imaginary part B of the input signal 201 canindependently take four values. The detailed structure of each systemcomponent can be the same as that illustrated in FIG. 4.

Next, effects of the communication system according to the presentembodiment are described. As has been described above, the spreadingcode generating apparatus 10 carries out M-sequence multiplication andzero value addition of the generated orthogonal codes to find channelidentification codes, and then carries out inverse Fourier transform ofthe channel identification codes, thereby generating the spreading codes121 and the inverse spreading codes 131. The transmitting apparatus 20and the receiving apparatus 30 store the spreading codes 121 and theinverse spreading codes 131, respectively, generated by the spreadingcode generating apparatus 10, and use a single code respectivelyselected from the stored codes to carry out spread spectrumcommunication.

The vectors of each orthogonal code generated by the spreading codegenerating apparatus 10 are orthogonal to each other. Sinceorthogonality is kept even after M-sequence multiplication and zerovalue addition, the channel identification codes obtained through thesetwo processes are also orthogonal to each other. Moreover, since inverseFourier transform is an orthogonal transform, the spreading codesobtained through inverse Fourier transform of the channel identificationcodes are orthogonal to each other. As such, by using spreading codesthat are orthogonal to each other, the communication system according tothe present embodiment can carry out spread spectrum communication asgeneral spread spectrum communication systems using PN codes or thelike.

Further, in the communication system according to the presentembodiment, vectors obtained through inverse Fourier transform of thechannel identification codes containing elements of 1 or −1 andorthogonal to each other are used as spreading codes. Therefore, whenthe spreading codes are subjected to Fourier transform, the elementsobtained as a result of Fourier transform become 1 or −1, and thus theirabsolute values are constant. Consequently, with the use of suchspreading codes for spreading, the input signal is spread over a widefrequency band with uniform power on the frequency axis. Such spectrumspreading with uniform power on the frequency axis improves transmissioncharacteristics and increases multiplexing capacity, compared with acase of conventional spread spectrum communication not achieving uniformpower on the frequency axis.

FIG. 5 is an illustration showing one example of transmissioncharacteristics of the communication system according to the presentembodiment. In FIG. 5, a horizontal axis indicates an S/N ratio(signal-to-noise ratio), and a vertical axis indicates a bit error rate.The transmission characteristics of the conventional art (denoted as abroken line) are observed when Hadamard codes each having a length of 16are used as the spreading codes. The transmission characteristics of thepresent embodiment (denoted as a solid line) are observed when the sameHadamard codes are inverse-Fourier-transformed and the resultant codesare used as the spreading codes. In both cases, a channel divisionscheme used is the CDMA (Code Division Multiple Access) scheme. Asillustrated in FIG. 5, in both cases, as the S/N ratio deteriorates, thebit error rate becomes larger. However, the characteristics of thepresent embodiment are more advantageous than those of conventional artin that the bit error rate is smaller. Therefore, according to thecommunication system of the present embodiment, it is possible toincrease multiplexing capacity in spread spectrum communication.

In the present embodiment, the spreading code generating apparatus 10generates Hadamard codes as the orthogonal codes, and generatesspreading codes based on the generated Hadamard codes. Alternatively,Walsh codes, Gold codes, or other codes may be used as the orthogonalcodes. Further, the spreading code generating apparatus 10 may generatechannel identification codes that are different from each other, and maygenerate spreading codes based on the generated channel identificationcodes. In this case, absolute values of elements of the channelidentification codes are preferably equal or approximately equal to eachother. Still further, the channel identification codes do notnecessarily have exact orthogonality, but may merely havecharacteristics so that an inner product of two vectors becomes closerto zero.

Also, in the present embodiment, the spreading code generating apparatus10 carries out M-sequence multiplication and zero value addition of thegenerated orthogonal codes. Alternatively, either or both of the twoprocesses may not be carried out. With this, as illustrated in FIGS.6A-6C, it is possible to construct a spreading code generating apparatus17 ((a) of FIG. 6) that carries out inverse Fourier transform of thegenerated orthogonal codes; a spreading code generating apparatus 18((b) of FIG. 6) that carries out multiplication of the generatedorthogonal codes, and the M-sequence and then inverse Fourier transform;and a spreading code generating apparatus 19 ((c) of FIG. 6) that addszero values to the generated orthogonal codes and then carries outinverse Fourier transform. These three types of spreading codegenerating apparatuses can achieve the same effects as those achieved bythe spreading code generating apparatus 10.

Further, in the present embodiment, the spreading code generatingapparatus 10 adds a predetermined number of zero values to the head andthe tail of each vector outputted from the multiplier 13. Alternativelyor simultaneously, a predetermined number of zero values may be insertedat the same position of each vector outputted from the multiplier 13.Such insertion of zero values at the same position of each vector canprovide a carrier hole (unused frequency band) on the frequency axis,thereby preventing another system from interfering with a carrier on thefrequency band. As such, the spreading code generating apparatus 10 mayadd or insert a predetermined number of zero values at the same positionin each different numerical value sequence.

Still further, in the present embodiment, the transmitting apparatus 20includes the RF modulating section 24 and the antenna 25, the receivingapparatus 30 includes the antenna 31 and the RF demodulating section 32,and wireless communication is performed between the transmittingapparatus 20 and the receiving apparatus 30. Alternatively, wiredcommunication is performed therebetween. In this case, the transmittingapparatus 20 may not carry out RF modulation, and the receivingapparatus 30 may not carry out RF demodulation.

As described above, the apparatus and method for spread spectrumcommunication uses vectors obtained by carrying out inverse Fouriertransform of different channel identification codes (orthogonal codes,for example) to carry out spectrum spreading with uniform power on thefrequency axis. Thus, it is possible to improve transmissioncharacteristics and increase multiplexing capacity.

1-8. (canceled)
 9. A method for transmitting a binary code using aFrequency Division Multiplexing (FDM), comprising: outputting one PNbinary code chosen from a plurality of PN binary codes; modulating aplurality of subcarriers in a band, one-by-one, by binary elementvalues, respectively, of the one PN binary code; multiplexing thesubcarriers having been modulated to obtain a transmission signal; andtransmitting the signal in an IFT (Inverse Fourier Transform) symboltime duration.
 10. The method according to claim 9, wherein, in saidmodulating step, subcarriers in one group or plural groups of successivesubcarriers in frequency domain are modulated by the binary elementvalues, respectively, of the one PN binary code.
 11. The methodaccording to claim 10, wherein, in said modulating step, the successivesubcarriers are modulated such that while preserving an unused frequencysubcarrier between the groups of successive carriers.
 12. The methodaccording to claim 9, wherein the PN binary code is used for identifyinga transmitting apparatus in a system operated by the FDM.
 13. Anapparatus for transmitting a binary code using a Frequency DivisionMultiplexing (FDM), said apparatus comprising: a processor that executesinstructions; and a memory device having said instructions storedthereon, wherein said instructions are issued for: outputting one PNbinary code chosen from a plurality of PN binary codes; modulating aplurality of subcarriers in a band, one-by-one, by binary elementvalues, respectively, of the one PN binary code; multiplexing thesubcarriers having been modulated to obtain a transmission signal; andtransmitting the signal in an IFT (Inverse Fourier Transform) symboltime duration.